In certain railroad highway grade crossing protection equipment, such as, in a fail-safe vehicle motion monitoring system, it is essential that the traffic and pedestrian warning signals and devices should not be capable of being deactivated when a moving train is approaching the highway or roadway crossing. It has been found that under certain unusual and adverse circumstances that the protection equipment may momentarily lose an approaching train. For example, it is possible to temporarily interrupt the warning devices, such as, the flashing lights at the highway grade crossing when a loss of the shunt by the wheels and axles causes the motion monitoring relay to become momentarily picked up and released. A similar situation occurs when a slowly approaching train enters the detection zone and encounters a bad bond within the detection zone which results in the temporary interruption of the flashing crossing relays and lights. In each case, the momentary loss is due to the fact that the reflected inpedance versus the track distance characteristic curve will undergo a downward step function which tends to cause an inflection of the change in the rate of the impedance and to simulate a slow to fast and then back to slow again approaching movement. It will be appreciated that while these momentary interruptions in the flashing lights do not effect the reliability of the protection system, it gives signal supervisors and engineers an uneasy feeling since they are accustomed to continuous operation of the warning devices. In order to alleviate the apprehensiveness of the signal personnel, it is simply necessary to supplement the fail-safe motion monitoring equipment with a suitable vital type of time delay circuit.
Accordingly, it is an object of this invention to provide a fail-safe time delay circuit for supplementing a railroad highway grade crossing motion monitoring apparatus.
Another object of this invention is to provide a vital type of time delay circuit for providing a time delay period for a motion monitoring device.
A further object of this invention is to provide a fail-safe timer for preventing a relay from picking up and dropping out due to the momentary appearance of an input signal.
Yet another object of this invention is to provide a unique timing circuit employing a pair of R-C networks for delaying the immediate energization of signal responsive device.
Yet a further object of this invention is to provide a novel time delay circuit having a detection relay for sensing the presence and absence of an input signal, a switching relay for assuming a picked-up and dropped-out condition in response to the presence and the absence of the input signal and associated electronic oscillating, gating and d. c. making circuits governed by the switching relay for providing a time delay period between the disappearance and the reappearance of the input signal prior to causing the pick up of the switching relay.
Still another object of this invention is to provide a fail-safe time delay circuit employing detection means for sensing the presence and absence of an input signal on a pair of terminals, switching means connectable between the pair of input signal terminals and a pair of d. c. supply terminals, the switching means assuming a first and a second condition in accordance with the presence and absence of the input signal on the input signal terminals, and associated oscillating, gating and d. c. making means controlled by the second condition of the switching means for providing a time delay period between the disappearance and the reappearance of the input signal on the pair of input signal terminals prior to the switching means reassuming the first condition.
Still a further object of this invention is to provide a new and improved fail-safe timer which is economical in cost, simple in construction, reliable in operation, durable in use and dependable in service.